In the solar cell technology invented by the late Dr. Praveen Chaudhari, a method is disclosed (U.S. Pat. No. 9,054,249) for making a tandem solar cell in which a “thin-silicon film can be used for heteroepitaxial deposition of other semiconductors, which might be more efficient converters of light to electricity.” The material “Perovskites,” although not new, has recently been the subject of a tremendous amount of attention in the solar cell technology community due to the quick progress and achievement of high efficiencies demonstrated with regard to light conversion for solar cell purposes over a relatively short period of time. The name ‘perovskite solar cell’ is derived from the ABX3 crystal structure of the absorber materials, which is referred to as perovskite structure. “Perovskites” is the nomenclature for any materials that adopt the same crystal structure as calcium titanate (ABX3). There are hundreds of different materials that adopt this structure, with a multitude of properties, including insulating, antiferromagnetic, piezoelectric, thermoelectric, semiconducting, conducting, and, probably most famously, superconducting. (H. Snaith “Perovskites: The Emergence of a New Era for Low Cost, High Efficiency Solar Cells”, 2013). Nonetheless, thousands of different chemical compositions are possible as perovskites are a wide ranging class of materials in which organic molecules made mostly of carbon and hydrogen bind with a metal such as lead and a halogen such as chlorine in a three dimensional crystal lattice.
Many believe that solar cells will need to have a power conversion efficiency (PCE) around 25% and a cost below $0.5/W to revolutionize how the world's population obtains its electricity. Perovskites' conversion efficiency has increased over the last five years from 4 percent to nearly 20 percent. The theoretical limit of perovskite's conversion efficiency is about 66 percent, compared to silicon's theoretical limit of about 32 percent. The ingredients used to create perovskite are widely available and inexpensive to combine, since it can be done at relatively low temperatures (around 100° C.). While there are many advantages to perovskites, there are also disadvantages. One of the components of the perovskite commonly experimented with—MAPbI—is Pb or lead—a highly toxic metal. And while perovskite based solar cells have not (yet) gained market entry, before they could do so any perovskite solar cells would have to undergo extensive testing to make sure that lead wouldn't be a risk factor. Although researchers have noted that the amount of lead present is relatively low, and would likely have a very minimal negative environmental impact, a perovskite without a toxic metal would be advantageous. Researchers have been able to produce lead-free perovskite cells that swap lead out for tin, which could eliminate the concern entirely. This tin (Sn) perovskite, when combined with another semiconductor material as a layer underneath for a tandem or multi-junction structure, could lead to an ideal non-toxic solar cell capable of solving current energy needs and to combating climate change. There is, however, another challenge. Since perovskite solar cells already have the efficiency that is needed for commercialization and can almost certainly be manufactured at a highly attractive cost, the primary barrier to commercialization is going to be obtaining long-term stability. The challenge appears to be that the films are highly reactive with water and have a tendency to emit methylammonium iodide. This also holds for perovskite/silicon tandem solar cells. As of the date of this disclosure, tandem solar cells with a lead based perovskite and crystalline silicon bottom layer have been fabricated and reported on. For example, methylammonium lead tri-halide perovskite and silicon solar cells can form a complementary pair. With the perovskite solar cell functioning as a top layer, it can harvest the short wavelength photons while the bottom layer coated with silicon is designed to absorb the long wavelength photons. As there are different wavelengths for solar energy, a combination of different materials for making solar cells would work best for energy absorption.
If a tandem solar panel could reach 30 percent efficiency, the impact on the balance-of-system cost could be enormous: only two thirds of the number of panels would be needed to produce the same amount of power as panels that are 20 percent efficient, greatly reducing the amount of roof space or land, installation materials, labor and equipment. (V. Sivaram et al “Outshining silicon . . . , Scientific American, July 2015”). The challenge is to produce good connections between semiconductors, something that has been challenging with regard to silicon because of the arrangement of silicon atoms in crystalline silicon (K. Bullis, “What Tech is Next for the Solar Industry” MIT Technology Review, 2013). The other material(s), on the silicon sublayer, also presents challenges. And in the case of perovskites film fabrication—how the film is made—is of crucial importance as it determines the film's texture, crystal structure, composition, and defect formation that collectively contribute toward over-all device performance. Furthermore, interface engineering has proven to effectively optimize device performance as it affects carrier dynamics across the entire device including charge generation, transportation, and collection. (Chen et al) As will be seen, the invention disclosed herein is directly related to all these issues.
The concept of the stacked solar cell was introduced to increase output voltage of a-Si:H solar cells. Only later it was recognized that stacked cells also offer a practical solution for improving the stabilized performance of a-Si:H based solar cells. Different terms such as tandem or dual junction or double junction solar cells are used in the literature to describe a cell in which two junctions are stacked on top of each other. A stack of three junctions is named a triple junction solar cell. The multi junction solar cell structure is far more complex than the single junction solar cell. For its successful operation there are two crucial requirements: (i) the current generated at the maximum power point has to be equal in each component cell (current matching) and (ii) an internal series connection between the component cells has to feature low electrical and optical losses. The internal series connection is accomplished at the p-n junction, where the recombination of oppositely charged carriers arriving from the adjacent component cells takes place. (M. Zeman “Advanced Amorphous Silicon Solar Cell Technologies”).
The requirement of current matching reflects the fact that component cells function as current sources which are connected in series. The component cell that generates the lowest current determines the net current flowing through the stacked two terminal cell. In order to avoid current losses, each component cell should generate the same current. The current generated by a component cell depends mainly on the absorption in the absorber layer of the cell, which is determined by the thickness of the absorber. Current is matched by adjusting the thickness of the absorber layer of each component cell. (M. Zeman “Advanced Amorphous Silicon Solar Cell Technologies”).
The tunnel recombination junction deals with the interface between the component cells. This interface is in fact an p-n diode. An ohmic contact between the component cells is required for proper operation of the stacked solar cell. The problem of obtaining the ohmic contact between the component cells can be resolved by fabricating a so-called tunnel recombination junction. This junction ensures that the electrons arriving at the n-type layer of the top cell and the holes arriving at the p-type layer of the bottom cell fully recombine at this junction. The recombination of the photogenerated carriers at this interface keeps the current flowing through the solar cell. A very high electric field in this reverse biased p-n junction facilitates tunneling of the carriers towards the defect states in the center of the junction. The effective recombination of the carriers takes place through these defective states. A tunnel recombination junction is usually realized by using microcrystalline silicon for at least one of the doped layers in order to obtain good ohmic contact. Another approach is to incorporate a thin oxide layer at the interface between the two component cells that serves as an efficient recombination layer. When the p-n junction functions as a good ohmic contact, the Voc of the stacked cell is the sum of the open circuit voltages of the component cells. (M. Zeman “Advanced Amorphous Silicon Solar Cell Technologies”).